Solubilities of noble metals in Fe-containing silicate melts as derived from experiments in Fe-free systems

The solubilities of noble metals (NM: Ir, Pd, Au, Pt, and Ru) in FeO-free s ilicate melts are known from recent experimental work. In this paper, calcu lations are presented that relate solubilities of NM in FeO-free melts to t hose in FeO-containing melts. The main difference between these two systems is the formation of Fe-NM alloys in FeO-containing melts. At f(O2) conditi ons of the QFM buffer and 1200 degreesC, binary alloys of Fe with Au, Ru, I r, Pd, or Pt containing 1, 2, 10, 17, and 25 at% Fe, respectively, are in t hermodynamic equilibrium with a silicate melt with 10 mol% FeO. Thus alloy formation leads to a significant reduction in the solubility of Pt and to a lesser reduction in the solubility of Pd. The effects for Ir and Ru are sm all, and for Au almost negligible. The reduction in solubilities depends on temperature, oxygen fugacity, and FeO content of the silicate melt. Format ion of FePt-alloys would lead to a preferred depletion of Pt in partial mel ts from the Earth's mantle, which, however, is not observed. One explanatio n is that mantle melting occurs under very oxidizing conditions (QFM+2). Th e calculations presented here should be considered a first step toward gain ing a better understanding of the behavior of NM during igneous processes.